Technical Abstract:
Laboratory studies have shown that Escherichia coli O157:H7 can be transferred to lettuce by a harvesting-coring knife. However, specific scientific data that reflect realistic contamination conditions needed for risk assessments are not available. Also needed are methods to prevent/reduce pathogen contamination. The main objectives of this project were to 1) determine soil pathogen levels required for pathogen transfer to the edible portions of lettuce via a contaminated coring knife; and 2) reduce the risk of pathogen transfer by improving coring knife design and disinfection. Series of experiments were performed with typical Salinas sandy-loam soil, and E. coli O157:H7 contamination levels ranging from 1 cfu/g to 100,000 cfu/g soil; worst case scenario and real life fresh-cut processing conditions were also modeled. Results indicate that detection of E. coli O157:H7 on lettuce transferred from soil via a contaminated knife is highly dependent on soil pathogen concentration, soil type and moisture level, and portion of lettuce sampled. Results also show that when coring knives are washed in chlorine alone, at least 50 ppm free residual chlorine is needed in wash solution to inactivate E. coli O15:H7 on welded knife parts. However, when ultrasound was combined with chlorine, pathogen inactivation to below detection levels on all knife parts was achieved with a much lower free chlorine level. Additionally, two prototype lettuce coring-knives were designed and tested; and results showed that they improved sanitation efficacy and reduced the potential for pathogen adherence.